Lithographic printing plates for offset printing have traditionally been produced using analog optical methods. These methods are time consuming, require wet processing and careful process control. Dry methods have been disclosed such as in U.S. Pat. No. 4,081,572 where a hydrophilic polymer layer is converted to a hydrophobic polymer imagewise. This method requires high energy photons such as those emitted by xenon flash lamps or relatively expensive gas lasers or doubled YAG lasers. It is not well suited for use with relatively inexpensive near IR diode lasers. There are examples, such as in U.S. Pat. No. 4,693,958, utilizing a single layer of polymer and absorber material where laser exposure chemically converts the polymer nature from hydrophilic to hydrophobic. In U.S. Pat. No. 4,034,183 a similar method is disclosed where a hydrophilic layer containing pigments is rendered hydrophobic when exposed to laser radiation and is used on a lithographic press without further processing. This process, however is relatively insensitive and impractical, requiring about 1 to 2 watts of laser power while exposing only 10 cm/sec. There are also photosensitive methods described that require traditional chemical processing as in U.S. Pat. Nos. 3,506,779; 4,020,762; 4,063,949.
Ablative methods have been disclosed where the top layer is etched from a plate to form relief patterns such as in U.S. Pat. Nos. 4,054,094 and 4,347,785. These methods require expensive extremely high power lasers. In other cases, as in U.S. Pat. No. 4,054,094, a hydrophilic surface is ablated to reveal an oleophilic underlayer. A similar approach was taken in U.S. Pat. Nos. 5,339,737 and 5,351,617 where a top coat is ablated and then wiped to expose an underlayer. These processes require two layers coated on a suitable substrate. One layer is ink receptive and the other wettable by fountain solution. At least one of the layers contains an absorber material either homogeneously mixed or heterogeneously dispersed therein. Intense near IR radiation from a focused laser causes ablation or loosening of the top layer. Debris left behind from incomplete ablation must be wiped or otherwise removed from the plate surface. For these applications a coating should be easily removable with modest laser exposure while unexposed areas must be tough enough to withstand normal press conditions. An improved ablation plate was disclosed in U.S. Ser. No. 08/614,437, now U.S. Pat. No. 5,605,780, that used a novel binder consisting of polymeric cyanoacrylate. No post treatment was necessary, however, removing the last traces of material can be difficult and exposure dependent. As a result background toning was sensitive to exposure conditions. Cyano containing polymers have also been recognized for their barrier properties in laser ablative imaging films as disclosed in U.S. Pat. No. 5,468,591, and as gas generating propellants in proofing systems as disclosed in U.S. Pat. No. 5,459,016, however, printing plate applications have special requirements and materials that work in one application do not necessarily work in others. Thus it does not follow that binder materials will work well in all three applications. To make an acceptable printing plate it is not sufficient that the transferred material be easily removed from the donor or that they are good propellants for other incorporated materials. Components, or their decomposition products, must have good adhesion to the receiver surface and good cohesive strength. Furthermore, transferred material must be relatively insoluble in press fluids such as ink and fountain solution and they must be abrasion insensitive for long run length.
Nitrocellulose, for example is a well known binder for ablation and material transfer applications it ablates well but does not hold up well to conventional printing press conditions when it has been transferred to a hydrophilic receiver such as anodized aluminum.
Material transfer methods for printing plate preparations are well known in the art, as disclosed for example in U.S. Pat. Nos. 3,945,318, and 3,964,389. In this method a donor sheet was placed in face-to-face contact with a receiver plate. The donor consisted of a coating on transparent Mylar.RTM. polyester film containing an absorber, such as carbon, an oleophilic material and a self oxidizing binder, such as nitrocellulose. In this disclosure, the hydrophilic receiver was a roughened anodized Al plate. A scanning focused laser was used to heat the donor imagewise. Intense rapid heating causes components of the donor film to be transferred to the receiver. Many other materials have been suggested for use as binders in transfer plate donors such as, phenol and cresol-formaldehyde resins (novalak resins), urea-formaldehyde, melamine-formaldehyde, alkyd resins, polyester resins, polyacrylate, polymethacrylate and polyethyacrylate, polyamindes (nylon), poly vinyl acetate, polyvinyl chloride, poly vinylidene chloride polystyrene, copolymers of styrene and butadiene, and poly alkylene-polyethylene as were disclosed in U.S. Pat. No. 3,962,513. Still others include methyl methacrylate, Butvar 76 (a reaction produce of poly (vinylalcohol and butyraldehyde)), alkyd resin, Cymel 301 (a melamine derivative), araldite 485-E50 (an epoxy resin), DeSoto 461-114 (a styrene-allyl alcohol copolymer) and novalac resin (cresol formaldehyde), as for example in U.S. Pat. No. 3,964,389; and vinylchloride and vinylacetate copolymer, Cymel (a UV crosslinkable polymer system), and hexamethoxymethylmelamine as disclosed in U.S. Pat. No. 4,626,493. Many of these binders, nitrocellulose for example, have been found to work quite poorly and must be supplemented with other transferable ink receptive components or layers to be useful on press. Under these extreme conditions some materials, will undergo reversible or irreversible decomposition. The prior art does not distinguish which among these many polymers produces plates with superior press characteristics.